egg minimum finance charge

gem4

I know its been covered before, and i've read the threads but cant get my head round the reasons.
I always pay my egg money balance in full and always well before the due date, (although i have a minimum payment DD in operation just incase I slip up).;)

I dont understand why i have this 50p minimum finance charge on my June statement. The due date of my statement was 13th June and the full payment was in my account by 3rd June. This is 10 days before the due date.

My card is used for food and petrol and very little else, certainly not for gambling or cash withdrawals....its simply to get the cashback. This is the first time i've had this charge. I cant see any reference to it on the egg website and am awaiting their response. Anyone got any suggestions? I hope this isnt going to be a regular occurrence